Certain known frequency measurement techniques use the transfer oscillator sampling approach and require two samplers. (See, for example, U.S. Pat. No. 3,836,758 entitled Wide Frequency Range Counter System Utilizing Automatic Range Searching and Loop Compensation issued on Sept. 17, 1974 and the Hewlett-Packard Journal, May 1974.)
The present invention requires only one sampler which is alternately driven by two signals of frequencies f.sub.1 and f.sub.2, which are sufficiently close by design to ensure both IF frequencies output by the sampler have the same harmonic number, satisfying the equations, EQU f.sub.x = Nf.sub.1 - IF.sub.1 EQU f.sub.x = Nf.sub.2 - IF.sub.2.
Measurement of the sampler output f using the phase continuous switching technique then yields an accurate weighted average of the IF frequencies from which f.sub.x may be calculated.
Phase continuous switching is defined as the technique where the transition from f.sub.1 as the driving frequency of the sampler to f.sub.2 as the driving frequency or vice versa is done only when the two frequencies f.sub.1 and f.sub.2 are in phase. A mixer is employed to supply the difference frequency f.sub.2 - f.sub.1 as the trigger source for the transitions between f.sub.1 driving the sampler and f.sub.2 driving the sampler.
The technique of phase continuous switching facilitates accurate measurement of the IF output while alternating rapidly between the two driving frequencies f.sub.1 and f.sub.2. Using a single sampler system without this ability, one had to measure the IF frequency through multiple gating (to avoid the spurious counts which occur when switching at random phase), and thereby introduced quantization error as well as repeated systematic gating errors. Alternatively, one had to switch frequencies between long intervals to allow time for sufficiently accurate measurement of each IF frequency. The latter approach requires that the input frequency must be sufficiently free from frequency drift and modulation to allow accurate results to be obtained from long periods of measurement time.